1. Field of the Invention
The present invention relates generally to bicycles and, more particularly, to a rear derailleur guard assembly for a bicycle.
2. Description of the Related Art
Bicycles, especially mountain bikes, or bicycles intended for off road use, are often equipped with a multi-speed drive train to provide a variety of available gear ratios to a rider of the bicycle. Due to the wide variety of terrain upon which a mountain bike may be ridden, in comparison to other types of bicycles, a relatively large range of gear ratios are often provided. For example, a very low gear ratio is beneficial when climbing a steep hill, while a much higher gear ratio permits the cyclist to travel at high speeds on flat ground or when going down a hill.
A chain and sprocket arrangement is a common type of drive train provided on current bicycles. A typical chain and sprocket arrangement includes a continuous drive chain interconnecting a selected one of a plurality of sprockets mounted to a pedal crank of the bicycle with a selected one of a plurality of rear sprockets drivingly connected to a rear wheel of the bicycle. Front and rear derailleurs are provided to move the drive chain to a selected one of the plurality of front and rear sprockets, respectively. Front and rear shifters control the front and rear derailleurs, respectively, and permit the rider of the bicycle to choose between the available gear ratios provided by the combination of front and rear sprockets.
As described above, the rear derailleur is configured to move the drive chain among the plurality of rear sprockets, or cogs. The rear derailleur typically includes a fixed portion mounted relative to the bicycle frame and a moveable portion, or cage. A pair of small sprockets, or pulleys, are supported at upper and lower ends of the cage and opposing sides of the pulleys engage the drive chain. As described, the rear shifter controls the rear derailleur to move the cage into alignment with a selected one of the plurality of rear cogs and the pulleys urge the drive chain into engagement with the selected cog.
In addition to moving the drive chain between cogs, the rear derailleur also operates to absorb slack in the drive chain, which increases with smaller gear ratios and decreases with larger gear ratios. The amount of slack taken up by the rear derailleur is influenced in part by the distance between the pair of pulleys and, thus, by the length of the derailleur cage. Accordingly, with the relatively large difference between the largest gear ratio and the smallest gear ratio found on most mountain bikes, the rear derailleur is often equipped with a relatively long derailleur cage.
The difference between the smallest and the largest gear ratios for a mountain bike is typically greater than the difference between the smallest and largest ratios for a bicycle intended primarily for use on the road. Accordingly, the derailleur cage is typically longer on a mountain bike than on a road bike. Thus, a mountain bike rear derailleur is much more likely to be damaged as a result of being struck by objects encountered while riding, such as rocks or sticks, than the derailleur cage of a road bike, due to the additional length of the derailleur cage.
The rear derailleur is typically mounted to a tab, or derailleur hanger, which extends from the bicycle frame to a position below the hub axis of the rear wheel. Typically, a derailleur mounting bolt supports the rear derailleur from the bicycle frame in a cantilevered arrangement. That is, the mounting bolt includes a threaded end, which engages a threaded aperture of the derailleur hanger, and a shaft portion, which extends outwardly from the derailleur hanger. The rear derailleur is supported on the shaft portion of the mounting bolt. Such an arrangement is highly susceptible to damage as a result of objects striking the derailleur cage. For example, the derailleur hanger may be bent, which may cause a misalignment between the derailleur cage and the plurality of cogs, thereby causing a reduction in shifting performance. In other cases, the derailleur mounting bolt may be broken, which may render the bicycle unridable.